1. Field of Invention
The present invention relates to a screen printing apparatus for pushing a printing material out of a printing head and printing the printing material on an object through a screen having one or more through-holes, and particularly to a method and an apparatus for controlling a pushing pressure applied to the printing material.
2. Related Art Statement
A conventional screen printing apparatus is disclosed in Japanese Patent Application laid open under Publication No. 8(1996)-48024. The screen printing apparatus includes (a) a printing-material container which accommodates, in a printing-material chamber thereof, a printing material and opens toward a screen and (b) a printing-material pusher which applies a pushing pressure to the printing material accommodated in the chamber and pushes the material out of the chamber. The pusher includes a pushing plate which is slideably fitted in the chamber and pushes the printing material out of the chamber, and a pushing air-cylinder device as a drive source that drives or moves the pushing plate in the chamber. The screen printing apparatus further includes a control device which controls the pressure of air supplied to the air cylinder, such that the pushing plate pushes, with a predetermined pushing pressure, the printing material out of the chamber. The pushing pressure is predetermined depending upon the sort and/or viscosity of the printing material used.
The above-described screen printing apparatus maintains the predetermined pushing pressure throughout one printing operation. However, the through-holes of the screen have different shapes, sizes and/or orientations, and are distributed at different densities. If the predetermined pressure is kept during one full printing operation, an insufficient amount of printing material may be printed on some portions of the object, and an excessive amount of printing material may be printed on other portions of the object. Thus, an uneven printing may occur to the object. For example, in the case where the printing material is squeezed into a through-hole having such a shape that the hole is wide in the direction of extension of the opening outlet of the printing-material chamber, a sufficient amount of printing material can be easily squeezed into the hole with a low pushing pressure. On the other hand, in the case of a through-hole having a shape that the hole is narrow in the above direction, there is such a tendency that a considerably high pushing pressure may be needed to squeeze a sufficient amount of printing material into the hole. Thus, in the case where a screen has a plurality of through-holes which have different shapes, sizes and/or orientations and/or are distributed at different densities (hereinafter, simply referred to as xe2x80x9ca complex printing patternxe2x80x9d), an uneven printing is likely to occur. Therefore, when a screen having a complex printing pattern is used, an operator needs to carefully adjust the pushing pressure. However, this adjusting operation takes a lot of time, and the quality of printing depends on the skill of the operator.
The present invention provides a pushing-pressure controlling method, a screen printing apparatus, and a recording medium which have one or more of the following technical features that are described below in respective paragraphs given parenthesized sequential numbers (1) to (21). Any technical feature that includes another technical feature shall do so by referring, at the beginning, to the parenthesized sequential number given to the latter feature. Thus, two or more of the following technical features may be combined, if appropriate. Each technical feature may be accompanied by a supplemental explanation, as needed. However, the following technical features and the appropriate combinations thereof are just examples to which the present invention is by no means limited. In addition, in the case where one technical feature recites a plurality of items, it is not essentially required that all of those items be simultaneously employed. That is, it is possible to select and employ only a portion (one, two, . . . , but not all) of those items.
(1) According to a first feature of the present invention, there is provided a method of controlling a pushing pressure which is applied to a printing material, such as a creamed solder, to push the material out of a printing head of a screen printing apparatus and print the material on an object, such as a printed wiring board, through at least one through-hole of a screen of the apparatus while at least one of the head and the screen is moved relative to the other of the head and the screen in a direction parallel to the screen, the method comprising the step of changing the pushing pressure applied to the printing material, while a printing operation is carried out on the object by the screen printing apparatus.
The pushing-pressure controlling method according to the first feature (1) can change the pushing pressure while one printing operation is carried out on one object. Therefore, the printing material can be printed, with an appropriate pushing pressure, on each portion of the object, depending upon, e.g., the shape of each through-hole of the screen. Thus, the quality of printing can be easily improved. However, it is difficult for an operator to change the pushing pressure during one printing operation.
Accordingly, it is preferred to prepare, before one printing operation, a command to change the pushing pressure, and automatically change, during the printing operation, the pushing pressure according to the prepared command. In addition, it is preferred to change the pushing pressure at a time when the printing material is not squeezed into any through-holes of the screen, that is, a pushing portion of the screen printing apparatus that pushes the printing material is moving above the screen but is not facing any through-holes of the screen, or when the material is being squeezed into the least number of through-holes. Moreover, it is preferred to complete changing the pushing pressure in a short time.
(2) According to a second feature of the present invention that includes the first feature (1), the step of changing the pushing pressure comprises changing the pushing pressure according to a predetermined pressure pattern representing a relationship between relative position of the printing head and the screen, and pressure.
The pushing-pressure controlling method according to the second feature (2) employs a predetermined pattern indicating the change of pushing pressure, and changes the pushing pressure according to the pattern. A shorter time is needed for an operator to prepare the pattern than a time needed for the operator to prepare one or more individual commands to change the pushing pressure. Thus, the efficiency of operation of the operator is improved. In addition, the quality of printing is stabilized irrespective of the degree of skill of the operator. It is preferred to prepare the pressure pattern based on, e.g., the shape(s) of the through-hole(s) of the screen, and it is preferred to prepare respective exclusive pressure patterns for individual screens.
(3) According to a third feature of the present invention that includes the second feature (2), the method further comprises the steps of inspecting a condition of the printing material printed on the object; and adjusting, based on a result of the inspection, the pushing pressure changed according to the predetermined pressure pattern.
The pushing-pressure controlling method according to the third feature (3) adjusts, based on the inspected condition of the printing material printed on the object, the pushing pressure for being used in another printing operation for another object. Therefore, even if the quality of printing performed in an initial period according to the predetermined pressure pattern may be not satisfactory, the quality of printing can be eventually improved and satisfied as the total number of objects printed increases. The condition of the printing material printed on the object may be at least one of (a) the amount of printing material printed on the object, (b) the shape of printing material printed on the object, and (c) the thickness of printing material printed on the object. The inspection of the condition of the printing material may be performed each time one printing operation for one object is finished, or for only a predetermined number of objects on each of which the printing material has been printed in an initial screen-printing period.
Alternatively, the inspection may be performed for each object until the inspection is performed for a predetermined number of objects in an initial period and it is ascertained that printing operations can be stably performed. Otherwise, the inspection may be performed for every second, third, object, i.e., every n-th object (xe2x80x9cnxe2x80x9d is a natural number not smaller than two).
(4) According to a fourth feature of the present invention, there is provided a method of controlling a pushing pressure which is applied to a printing material to push the material out of a printing head of a screen printing apparatus and print the material on an object through at least one through-hole of a screen of the apparatus, while at least one of the head and the screen is moved relative to the other of the head and the screen in a direction parallel to the screen, the method comprising the steps of inspecting a condition of the printing material printed on the object; and automatically adjusting, based on a result of the inspection, the pushing pressure applied to the printing material.
The explanation made for the third feature (3) is true with the pushing-pressure controlling method according to the fourth feature (4). However, according to the present method, the pushing pressure may be changed during one printing operation or may be maintained at a prescribed value.
(5) According to a fifth feature of the present invention that includes the third or fourth feature (3) or (4), the step of inspecting the condition comprises taking an image of the printing material printed on the object.
If an image taking device takes an image of the printing material printed on the object and a data processor (e.g., a computer) processes a batch of image data representing the taken image, the condition of the printing material can be quickly inspected.
(6) According to a sixth feature of the present invention that includes the fifth feature (5), the step of taking the image comprises emitting a planar light toward the object and the printing material printed on the object and taking an image of respective portions of the object and the printing material that are exposed to the planar light.
If the planar light is incident to respective plane surfaces of the object and the printing material, respective linear portions (i.e., narrow, belt-like portions) of the object and the material are exposed to the planar light, and an image including the light linear portions of the object and the material and the dark remaining portions of the object and the material is taken. However, since the printing material printed on the object projects from the upper surface of the object and has a three-dimensional shape, the respective exposed linear portions of the object and the material are separated from each other in a three-dimensional space. If the above image is taken in a direction intersecting the planar light, a two-dimensional image including the two or more light linear portions can be taken. This two-dimensional image includes information representing the thickness of the printing material. Thus, information representing, e.g., the thickness or volume of the printing material printed on the object can be obtained by processing one or more batches of image data representing one or more two-dimensional images thus taken. It is preferred to process a plurality of batches of image data representing a plurality of two-dimensional images. If a greater number of two-dimensional images are processed, information approximating the three-dimensional image of the printing material can be obtained. Thus, a three-dimensional shape and three dimensions of the printing material, and/or positions of the printing material on the object can be obtained. It is possible to emit simultaneously a plurality of planar lights toward the object and take simultaneously a plurality of two-dimensional images of the object and the printing material. Alternatively, as will be described in the following feature (7), it is possible to move at least one of an image taking device (and a lighting device) and the object relative to the other of the image taking device (and the lighting device) and the object in a direction parallel to the upper surface of the object and take a plurality of two-dimensional images at different timings.
(7) According to a seventh feature of the present invention that includes the sixth feature (6), the step of inspecting the condition comprises taking respective images of a plurality of portions of the object and a plurality of portions of the printing material printed on a surface of the object that are sequentially exposed to the planar light when at least one of the planar light and the object is moved relative to the other of the planar light and the object in a direction parallel to the surface of the object; and calculating, based on the taken images, an amount of the printing material printed on the object.
The method according to the seventh feature (7) can determine the amount or volume of the printing material by processing only a less amount of image data than the entire batch of image data representing the entire surface of the object, because the respective images of the respective light portions of the object and the printing material are formed within a considerably narrow area only in the entire vision field of the image taking device that can cover the entire surface of the object. Therefore, the condition of the printing material can be inspected in a short time and accordingly the pressing pressure can be adjusted in a short time. This processing method is performed by a three-dimensional-data obtaining apparatus disclosed in Japanese Patent Application No. 11(1999)-226587 and the corresponding U.S. Patent Application assigned to the assignee of the present application.
(8) According to an eighth feature of the present invention, there is provided a screen printing apparatus for printing a printing material on an object through a screen having at least one through-hole, comprising a printing head which includes (A) a printing-material container having (a1) a printing-material chamber in which the printing material is accommodated, and (a2) a printing-material outlet which communicates with the printing-material chamber and opens toward the screen, and (B) a printing-material pushing device which applies a pushing pressure to the printing material accommodated in the printing-material chamber to push the material out of the chamber; a moving device which moves at least one of the printing head and a combination of the screen and the object relative to the other of the head and the combination, in a direction parallel to the screen; and a pushing-pressure control device which controls the pushing pressure applied to the printing material by the printing material pushing device and which includes a pushing-pressure changing portion which changes the pushing pressure applied to the printing material while a printing operation is carried out on the object by the screen printing apparatus.
The screen printing apparatus according to the eighth feature (8) can perform the method according to the first feature (1). The control of the pushing pressure may be a so-called feed-back control in which an actual pushing pressure is detected and controlled, or a so-called open-loop control in which an actual pushing pressure is not detected.
(9) According to a ninth feature of the present invention that includes the eighth feature (8), the printing-material outlet linearly extends in a direction intersecting the direction in which the moving device moves the one of the printing head and the combination of the screen and the object relative to the other of the head and the combination, the printing-material chamber has a length substantially equal to a length of the outlet, and the printing-material pushing device includes a pushing member which is substantially fluid-tightly fitted in the printing-material chamber and a pushing-member drive device which moves the pushing member in the chamber toward the outlet.
In the case where the printing-material outlet is short, it is possible to apply a pressurized gas directly to the printing material accommodated in the printing-material chamber and control the pushing pressure by controlling the pressure of the pressurized gas. However, if this method is employed in the case where the outlet is long, a gas passage may be produced or created through the printing material when the remaining amount of the printing material in the printing-material chamber decreases, so that the pressurized gas blows out of the outlet, thereby making it impossible to continue printing. This problem may be solved by providing a pressing piston in a pressing chamber which has a small cross section and communicates with the printing-material chamber, and moving the pressing piston in the pressing chamber to equally pressurize all the printing material accommodated in the chamber. However, since, generally, a printing material has a high viscosity, the pressing piston cannot equally apply the pressure over the entirety of the long outlet, and accordingly the pushing pressure cannot be effectively controlled. In contrast, the screen printing apparatus according to the ninth feature (9) can uniformly apply the pushing pressure to the printing material over the entirety of the long printing-material outlet, and can effectively control the pushing pressure over the entirety of the long outlet.
(10) According to a tenth feature of the present invention that includes the ninth feature (9), the pushing-member drive device comprises a pressurized-air-operated cylinder device, and the pushing-pressure control device comprises a pressure control device which controls a pressure of a pressurized air supplied to the cylinder device.
The screen printing apparatus according to the tenth feature (10) employs the pressurized-air-operated cylinder device (so-called xe2x80x9cair cylinderxe2x80x9d) as the drive source that drives or moves the pushing member, and accordingly the pushing-member drive device can be produced with ease and at low cost. In addition, the control of the pressure of the pressurized air supplied to the air cylinder is easy.
(11) According to an eleventh feature of the present invention that includes any one of the eighth to tenth features (8) to (10), the pushing-pressure control device comprises a pattern-using control portion which changes the pushing pressure applied to the printing material according to a predetermined pressure pattern representing a relationship between relative position of the printing head and the screen, and pressure.
The screen printing apparatus according to the eleventh feature (11) can carry out the method according to the second feature (2).
(12) According to a twelfth feature of the present invention that includes the eleventh feature (11), the apparatus further comprises a condition inspecting device which inspects a condition of the printed material printed on the object, and the pushing-pressure control device further comprises a pushing-pressure adjusting portion which adjusts, based on a result of the inspection, the pushing pressure changed by the pattern-using control portion.
The screen printing apparatus according to the twelfth feature (12) can carry out the method according to the third feature (3).
(13) According to a thirteenth feature of the present invention, there is provided a screen printing apparatus for printing a printing material on an object through a screen having at least one through-hole, comprising a printing head which includes (A) a printing-material container having (a1) a printing-material chamber in which the printing material is accommodated, and (a2) a printing-material outlet which communicates with the printing-material chamber and opens toward the screen, and (B) a printing-material pushing device which applies a pushing pressure to the printing material accommodated in the printing-material chamber to push the material out of the chamber; a moving device which moves at least one of the printing head and a combination of the screen and the object relative to the other of the head and the combination, in a direction parallel to the screen; a condition inspecting device which inspects a condition of the printed material printed on the object; and a pushing-pressure control device which controls, based on a result of the inspection, the pushing pressure applied to the printing material by the printing-material pushing device.
The screen printing apparatus according to the thirteenth feature (13) can carry out the method according to the fourth feature (4).
(14) According to a fourteenth feature of the present invention that includes the twelfth or thirteenth feature (12) or (13), the condition inspecting device comprises an image taking device which takes an image of the printing material printed on the object.
(15) According to a fifteenth feature of the present invention that includes the fourteenth feature (14), the condition inspecting device further comprises an amount calculating device which calculates, based on a batch of image data which is produced by the image taking device and represents the taken image, an amount of the printing material printed on the object.
(16) According to a sixteenth feature of the present invention, there is provided a recording medium for recording a program such that the program is readable by a computer to control a pushing pressure applied to a printing material to push the material toward a screen having at least one through-hole and print the material on an object through the through-hole of the screen, the program comprising the step of changing the pushing pressure applied to the printing material, depending on a position where the printing material is printed on the object.
The recording medium according to the sixteenth feature (16) can carry out the method according to the first feature (1).
(17) According to a seventeenth feature of the present invention, there is provided a recording medium for recording a program such that the program is readable by a computer to control a pushing pressure applied to a printing material to push the material out of a printing head of a screen printing apparatus and print the material on an object through at least one through-hole of a screen of the apparatus, while at least one of the printing head and the screen is moved relative to the other of the head and the screen in a direction parallel to the screen, the program comprising the steps of obtaining a basic information needed to control the pushing pressure; and controlling the pushing pressure applied to the printing material, based on the obtained basic information.
The basic information may include at least one information selected from the group consisting of a predetermined pressure pattern representing a relationship between relative position of the printing head and the screen, and pressure, and an inspection result obtained by inspecting a condition of the printing material printed on the object.
(18) According to an eighteenth feature of the present invention that includes the seventeenth feature (17), the basic information comprises information indicating that the pushing pressure is changed depending upon a position where the printing material is printed on the screen.
The recording medium according to the eighteenth feature (18) may employ the second or third feature (2) or (3).
(19) According to a nineteenth feature of the present invention that includes the seventeenth or eighteenth feature (17) or (18), the step of obtaining the basic information comprises obtaining, as the basic information, information relating to a condition of the printing material printed on the object.
The recording medium according to the nineteenth feature (19) may employ any one of the fifth to seventh features (5) to (7).
(20) According to a twentieth feature of the present invention, there is provided a recording medium for recording a program such that the program is readable by a computer to control a pushing pressure applied to a printing material to push the material toward a screen having at least one through-hole and print the material on an object through the through-hole of the screen, the program comprising the steps of obtaining a basic information needed to control the pushing pressure; and controlling the pushing pressure applied to the printing material, based on the obtained basic information.
The recording medium according to the twentieth feature (20) may employ any one of the first to seventh features (1) to (7).
(21) According to a twentieth feature of the present invention, there is provided a method of controlling a speed at which at least one of a printing head and a screen of a screen printing apparatus is moved relative to the other of the head and the screen, in a direction parallel to the screen, while a printing material is pushed out of the head and is printed on an object through at least one through-hole of the screen, comprising the step of changing the speed while a printing operation is carried out on the object by the screen printing apparatus.
In addition to, or in place of, the changing of the pushing pressure according to any one of the above-described features, the present method is for changing the speed at which at least one of the printing head and the screen is moved relative to the other of the head and the screen. Thus, the present method can improve the quality of printing of a complex pattern including different portions whose through-holes have different shapes, sizes, and/or orientations and are distributed at different densities. In the case where the pushing pressure is not changed, more printing material can be squeezed into the through-holes when the speed of relative movement of the printing head and the screen is lower. Therefore, if the relative-movement speed is low for the portion(s) where a printing defect is likely to occur and is high for the portion(s) where the defect is unlikely, the quality of printing can be improved while lowering of the efficiency of printing is prevented.